1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus equipped with the same. The present invention can suitably applied to an image pickup apparatus using a solid state image pickup element, such as a video camera, electronic still camera, broadcast camera and surveillance camera and to an image pickup apparatus such as a camera using a silver halide film.
2. Description of the Related Art
Positive-lead type zoom lenses in which a first lens unit located closest to the object side has a positive refractive power have been known.
U.S. Pat. No. 7,206,137 discloses a zoom lens composed of four lens units having, in order from the object side to the image side, positive, negative, positive, and positive refractive powers respectively and a zoom lens composed of five lens units having, in order from the object side to the image side, positive, negative, positive, positive, and positive refractive powers respectively. The zoom lenses disclosed in U.S. Pat. No. 7,206,137 has a zoom ratio not lower than 15 and a field angle of approximately 62 degrees at the wide angle end. U.S. Pat. No. 7,177,092 discloses a zoom lens composed of five lens units including, in order from the object side, positive, negative, positive, negative, and positive refractive powers respectively. The zoom lens disclosed in U.S. Pat. No. 7,177,092 has a zoom ratio of approximately 10 and a field angle of approximately 76 degrees at the wide angle end.
In order to enables the retraction of a zoom lens into a camera body thereby achieve slimming of the camera when not in use, it is generally effective to increase the refractive power (or the optical power, which is equal to the reciprocal of the focal length) of the lens units that constitute the zoom lens high and to reduce the number of lenses in each lens unit. However, if a zoom lens is designed in this way, an increase in the thickness of lenses will be necessitated with an increase in the refractive power of lens surfaces in order for each lens to have an appropriate edge thickness, an increase in the front lens diameter in particular will result, and satisfactory reduction in the lens size will fail to be achieved. Moreover, difficulty in correction of aberration such as chromatic aberration at the telephoto end will also result.
To achieve a high zoom ratio and size reduction at the same time, it is important to optimize the refractive power of the lens units, the overall length of the zoom lens, and the imaging magnification of the zooming lens unit. In the aforementioned four-unit and five-unit zoom lenses, it will be difficult to achieve good optical performance while achieving a high zoom ratio higher than 10 and wide field angle, unless the refractive power of the lens units is appropriately set.
An object of the present invention is to provide a zoom lens that is slim in the lens thickness when in a retracted state, has a wide field angle and high zoom ratio, and achieves high optical performance throughout the entire zoom range. Another object of the present invention is to provide an image pickup apparatus equipped with such a zoom lens.